Hydrolysis in water or in aqueous hydrochloric acid of .alpha.,.alpha.,.alpha..alpha.',.alpha.'-pentachloro-o-xylene to prepare phthalaldehydic acid proceeds very slowly and requires several days. Alkaline hydrolysis is not suitable because of the presence of an aldehydic group in the desired reaction product, which would then undergo more or less extensive polymerization with concomitant formation of condensation by-products and colored substances difficult to eliminate.
The hydrolysis rate is considerably increased by the use of a catalyst. For instance, U.S. Pat. No. 2,748,161 describes a process of catalytic hydrolysis of pentachloroxylene in the presence of an aromatic sulfonic acid selected from sulfonated aromatic hydrocarbons of the benzene and naphthalene series and the sulfonated nuclear-halogenated derivatives thereof, at 90.degree.-120.degree. C. On the other hand, U.S. Pat. No. 2,748,162 describes the hydrolysis of pentachloroxylene in aqueous hydrohalic acid in the presence of at least one metal halide selected from zinc chloride, zinc bromide, ferric chloride, ferric bromide, cadmium chloride and cadmium bromide at 100.degree.-150.degree. C. These processes give acceptable yields of phthalaldehydic acid, but the presence of large amounts of the employed catalyst in the reaction mixture creates severe technical difficulties of separation, particularly when the manufacture is carried out on an industrial scale. Moreover, the presence of metallic ions, such as Zn.sup.2.sup.+, Fe.sup.3.sup.+ and Cd.sup.2.sup.+, ions in the waste creates serious ecological problems.
On the other hand, the absence of impurities from phthalaldehydic acid is essential if it is to be used in organic light-sensitive compositions, such as those described in British Pat. No. 1,170,265 or in French Pat. No. 2,016,397. A high-purity standard is also of extreme importance if phthalaldehydic acid is to be employed as the starting material for manufacturing 1(2H)-phthalazinone, a substance of high commercial utility both as a component of light-sensitive compositions, such as those described in U.S. Pat. No. 3,682,684 and in German Pat. Nos. 1,908,758, 2,042,054 and 2,139,252, and as an intermediate for a number of organic substances of much use in all fields of technology. Thus, if phthalaldehydic acid is reacted with hydrazine to prepare 1(2H)-phthalazinone, or with hydrazine derivatives such as semicarbazide or thiosemicarbazide, the presence of trace amounts of metals in the starting phthalaldehydic acid may catalyze the decomposition of those reactants, in some instances accompanied by explosions. As a matter of fact, heavy metals in the form of their salts and oxides, particularly ferric, cupric, molybdic and chromic ions, are known to favor catalytically that decomposition; see J. E. Troyan, "Properties, Production and Uses of Hydrazine", Industrial Engineering Chemistry, 45, 2608 (1953).